1. Technical Field
This application claims priority based on Japanese Patent Application No. 2007-301114 filed on Nov. 21, 2007 in Japan, the entire contents of which are hereby incorporated in the present specification by reference.
The present invention relates to a piezoelectric filter using a piezoelectric vibrator and a method for manufacturing such a filter.
2. Background Art
There have been demands for small-size and light-weight parts as built-in parts to be used in an electronic apparatus such as a portable apparatus. For example, as a filter for use in a portable apparatus, a small-size filter, as well as a filter whose frequency characteristic can be precisely adjusted, has been required. A filter using a piezoelectric vibrator has been known as one of the filters that satisfy these demands.
Conventional piezoelectric vibrator will be described with reference to FIGS. 14A to 14C. FIG. 14A is a cross-sectional view that shows a basic structure of a conventional piezoelectric vibrator 60. The piezoelectric vibrator 60 has a resonance unit having a structure in which a piezoelectric element 61 is sandwiched by a lower electrode portion 62 and an upper electrode portion 63. This resonance unit is mounted on a substrate 65 with a cavity 64 formed therein. This cavity 64 can be formed by partially etching the substrate 65 from its rear face by using a micro-processing method.
As shown in FIG. 14B, the piezoelectric vibrator 60 applies an electric field in a thickness direction to the piezoelectric element 61 by the lower electrode portion 62 and the upper electrode portion 63 so that vibrations in the thickness direction are generated. The following description will discuss operations of the piezoelectric vibrator 60 by reference to thickness longitudinal vibrations of an endless flat plate. In the piezoelectric vibrator 60, when an electric field is applied between the lower electrode portion 62 and the upper electrode portion 63, electric energy is converted to mechanical energy in the piezoelectric element 61. The induced mechanical vibrations are extending vibrations in the thickness direction, and cause expansions and shrinkages in the same direction as that of the electric field. In general, the piezoelectric vibrator 60 is operated by utilizing resonance vibrations in the thickness direction of the piezoelectric element 61 to give resonance having a frequency the half-wavelength of which corresponds to the thickness. The cavity 64 shown in FIG. 14A is utilized so as to ensure the thickness longitudinal vibrations of the piezoelectric element 61.
As shown in FIG. 14C (a), the equivalent circuit of the piezoelectric vibrator 60 is allowed to have both of series resonance and parallel resonance. More specifically, the circuit is formed by a series resonator unit configured by a capacitor C1, an inductor L1 and a resistor R1 and a capacitor C0 that is connected in parallel to the series resonance unit. By using this circuit structure, the admittance frequency characteristic of the equivalent circuit has a maximum admittance at a resonance frequency fr and a minimum admittance at an anti-resonance frequency fa, as shown in FIG. 14C (b). Here, the resonance frequency fr and the anti-resonance frequency fa have the following relationships.fr=1/{2π√(L1×C1)}fa=fr√(1+C1/C0)
In the case where the piezoelectric vibrator 60 having such an admittance frequency characteristic is applied as a filter, since the resonance vibrations of the piezoelectric element 61 are utilized, a small size filter with a low loss can be achieved. As shown in FIG. 15, two piezoelectric vibrators 71 and 72 are connected in series with, as well as in parallel with each other so that, as shown in FIG. 16, a band-pass filter that allows the resonance frequency of the series piezoelectric vibrators and the anti-resonance frequency of the parallel piezoelectric vibrators to be made virtually coincident with each other can be easily configured. However, in order to make the frequencies coincident with each other, it is necessary to design the parallel piezoelectric vibrators so as to have a frequency lower than that of the series piezoelectric vibrators, as a whole. Here, Japanese Patent Laid-open Publication No. 2002-335141, Japanese Patent Laid-open Publication No. 2005-223479 and the like have disclosed inventions for carrying out such frequency adjustments.
Referring to FIG. 17, the following description will discuss a conventional frequency-adjusting method disclosed in Japanese Patent Laid-open Publication No. 2002-335141. This method uses a general mass load structure as one of frequency-adjusting methods. FIG. 17 is a cross-sectional view that shows a structure of a piezoelectric filter using two piezoelectric vibrators.
In order to manufacture resonators 140 and 150 on a substrate 132, a first bottom electrode 142 and a second bottom electrode 152 are formed, and these electrodes respectively bridge over a first void 141 and a second void 151. Next, a piezoelectric (PZ) layer 134 is formed over both of the first and second bottom electrodes 142 and 152, and the PZ layer 134 has a first portion 144 located on the first bottom electrode 142 and a second portion 154 located on the second bottom electrode 152. Next, a surface electrode layer 136 is formed, and the surface electrode 136 has a first section 146 formed on the first portion 144 and a second section 156 formed on the second portion 154. Next, a surface load film 138 is formed on the first section 146 so as to desirably cover the entire first section 146. The surface load film 138 includes a conductive material or an insulating material, or both of these, and although not particularly limited, the material includes molybdenum, aluminum nitride, or silicon dioxide. Next, the surface load film 138 is over-etched to form a first surface electrode (combination of the etched surface load film 148 and the first section 146 (148+146)). In other words, the surface load film 138 and the surface electrode layer 136 are simultaneously etched to form the first surface electrode (148+146). A second surface electrode 156 can be made by the same processes as those for forming the first surface electrode (148+146). Since no load electrode exists on the second surface section 156 of the surface electrode layer 136, the second surface electrode 156 is formed, with the second section 156 being left, while the surface electrode layer 136 is etched so as to eliminate all the other portions of the surface electrode layer 136, with the first surface electrodes (148+146) being left. With this structure, in the first resonator 140, a greater mass load is applied thereto in comparison with that applied on the second resonator 150 because of the portion corresponding to the surface load film 148. Thus, the first resonator 140 has a reduction in the frequency so that the first resonator 140 and the second resonator 150 are made different in their frequencies.
Next, FIG. 18 shows another frequency adjusting method disclosed in Japanese Patent Laid-open Publication No. 2005-223479. In the same manner as in Japanese Patent Laid-open Publication No. 2002-335141, this method uses a general mass load structure as one of the frequency adjusting methods. FIG. 18 is a cross-sectional view showing a piezoelectric filter using two piezoelectric vibrators. In FIG. 18, a formation area for a first thin-film bulk vibrator 111 is referred to as a first area, and a formation area for a second thin-film bulk vibrator 112 is referred to as a second area. The diaphragm structures of the thin-film bulk vibrators are respectively formed on voids 109 and 110 formed on the rear face side of a single substrate, and include a base layer 102 of the first area and a base layer 121 of the second area, a lower electrode layer 103 of the first area and a lower electrode layer 104 of the second area, a piezoelectric layer 105 of the first area and a piezoelectric layer 106 of the second area, and an upper electrode layer 107 of the first area and an upper electrode layer 108 of the second area. The film thickness t2 of the base layer 121 of the diaphragm structure of the second area is made thicker than the film thickness t1 of the base layer 102 of the diaphragm structure of the first area. Therefore, the film thickness T2 of the diaphragm structure of the second area including the base layer 121 is thicker than the film thickness T1 of the diaphragm structure of the first area including the base layer 102, and consequently, the resonance frequency of the second thin-film bulk vibrator 112 is made lower than the resonance frequency of the first thin-film bulk vibrator 111.
In the inventions disclosed in Japanese Patent Laid-open Publication No. 2002-335141 and Japanese Patent Laid-open Publication No. 2005-223479, a desired resonance frequency is achieved by adding a mass load effect to a piezoelectric vibrator, and a filter can be formed by realizing piezoelectric vibrators having a plurality of different resonance frequencies.
However, in the piezoelectric vibrators disclosed in Patent Documents 1 and 2, a load film is formed only on one face of an optimal piezoelectric vibrator shown in FIG. 19A so as to carry out the frequency adjustment. FIG. 19B shows a structure in which a load film 85 is formed beneath a lower electrode 82. In the optimal piezoelectric vibrator shown in FIG. 19A, since the two faces in the vertical direction are made in contact with the free space, the two faces serve as free ends so that on the basic mode, vibrations are exerted with ½ of a wavelength, with the center of a piezoelectric element 81 in the thickness direction serving as the node of the vibrations. At this time, energy to be used in the piezoelectric element 81 is maximized, and the effective coupling coefficient of the piezoelectric vibrator is also maximized. However, as shown in FIG. 19B, in the case where the load film 85 is formed only on one of the faces, the node of vibrations is changed in the direction in which the load film is formed. At this time, the energy to be utilized in the piezoelectric element 81 is reduced, with the result that the coupling coefficient deteriorates. FIG. 19C shows the results of numeric calculations of the size of the coupling coefficient relative to the film thickness of the load film. The axis of abscissas indicates the thickness of the load film 85 standardized by the film thickness of the piezoelectric element 81, and the axis of ordinates indicates the coupling coefficient. Here, AlN is used as the piezoelectric element, Mo is used as the electrode, and SiO2 is used as the load film. As shown in FIG. 19C, it is found that as the thickness of the load film 85 becomes greater, the coupling coefficient greatly deteriorates. In the case where this is used in a W-CDMA system (2 GHz band) of a portable telephone, the thickness of the piezoelectric element is about 1 μm, and a desired amount of frequency adjustment (thickness of the load film) is about 0.5 μm, with the result that the coupling coefficient is lowered by about 15% in comparison with that of the optimal piezoelectric vibrator.
In FIGS. 19A-19C, the explanation is given by exemplifying a structure in which the load film is formed only beneath the lower electrode; however, the same results are obtained in the case where the load film is formed only on the upper electrode.
Therefore, an object of the present invention is to provide a frequency adjusting method in which the degree of degradation of the coupling coefficient is improved.
Moreover, as a method for manufacturing a conventional thin-film bulk acoustic wave resonator (FBAR: Film Bulk Acoustic wave Resonator) filter, a manufacturing method using a transferring technique has been disclosed. When the conventional frequency adjusting methods of Patent Documents 1 and 2 are applied to this manufacturing method, degradation of the yield occurs.
Therefore, another object of the present invention is to provide a frequency adjusting method by which the degree of degradation of the coupling coefficient is suppressed. Moreover, still another object is to provide a method for suppressing the degradation of the yield in the manufacturing method using the transferring technique.
A piezoelectric filter having a first structure of the present invention is provided with: first and second piezoelectric vibrators, each having: a substrate; a lower load film formed on the substrate; a lower electrode formed on the lower load film; a piezoelectric element formed on the lower electrode; an upper electrode formed on the piezoelectric element; and an upper load film formed on the upper electrode, wherein the piezoelectric filter is formed by electrically connecting the first and second piezoelectric vibrators. In this structure, resonance frequencies of the first and second piezoelectric vibrators are adjusted by the respective lower load film and upper load film so that the resonance frequencies of the first and second piezoelectric vibrators are made different from each other.
A method for manufacturing a piezoelectric filter having a first structure of the present invention includes the steps of: forming an upper electrode on one of main faces of a piezoelectric element; forming a lower electrode on the other main face of the piezoelectric element; forming an upper load film on a face opposing the face of the upper electrode on which the piezoelectric element is formed; and forming a lower load film on a face opposing the face of the lower electrode on which the piezoelectric element is formed, and this method is characterized in that, in the first and second piezoelectric vibrators to be formed, the thicknesses of the respective upper load film and lower load film are adjusted so that the resonance frequencies of the first and second piezoelectric vibrators are made different from each other.
A piezoelectric filter having a second structure of the present invention is provided with: a first piezoelectric vibrator formed with a first area of a piezoelectric element being interposed therebetween, and a second piezoelectric vibrator formed with a second area of the piezoelectric element being interposed therebetween, which are electrically connected to each other, and in this structure,
the first piezoelectric vibrator is provided with:
a lower electrode formed on one of main faces of the first area of the piezoelectric element;
a lower load film formed on a face opposing a face of the lower electrode that is made in contact with the piezoelectric element;
an upper electrode formed on the other main face of the first area of the piezoelectric element; and
an upper load film formed on a face opposing a face of the upper electrode that is made in contact with the piezoelectric element, and
the second piezoelectric vibrator is provided with:
a lower electrode formed on one of main faces of the second area of the piezoelectric element;
a lower load film formed on a face opposing a face of the lower electrode that is made in contact with the piezoelectric element;
an upper electrode formed on the other main face of the second area of the piezoelectric element; and
an upper load film formed on a face opposing a face of the upper electrode that is made in contact with the piezoelectric element, and
the piezoelectric filter is characterized in that resonance frequencies of the first and second piezoelectric vibrators are adjusted by the respective lower load films and upper load films of the first piezoelectric vibrator and the second piezoelectric vibrator so that the resonance frequency of the first piezoelectric vibrator and the resonance frequency of the second piezoelectric vibrator are made different from each other.
A method for manufacturing a piezoelectric filter a second structure of the present invention is directed to a method for manufacturing a piezoelectric filter provided with: a first piezoelectric vibrator formed with a first area of a piezoelectric element being interposed therebetween, and a second piezoelectric vibrator formed with a second area of the piezoelectric element being interposed therebetween,
wherein the step of forming the first piezoelectric vibrator, with the first area of the piezoelectric element being interposed therebetween, further includes:
forming an upper electrode on one of main faces of the first area of the piezoelectric element;
forming a lower electrode on the other main face of the first area of the piezoelectric element;
forming an upper load film on a face opposing a face of the upper electrode that is made in contact with the piezoelectric element; and
forming a lower load film on a face opposing a face of the lower electrode that is made in contact with the piezoelectric element, and
wherein the step of forming the second piezoelectric vibrator, with the second area of the piezoelectric element being interposed therebetween, further includes:
forming an upper electrode on one of main faces of the second area of the piezoelectric element;
forming a lower electrode on the other main face of the second area of the piezoelectric element;
forming an upper load film on a face opposing a face of the upper electrode that is made in contact with the piezoelectric element; and
forming a lower load film on a face opposing a face of the lower electrode that is made in contact with the piezoelectric element, and
wherein, in the step of forming the respective upper load film or lower load film of the first piezoelectric vibrator and the second piezoelectric vibrator, or in the succeeding step thereof, the thicknesses of the upper load film and the lower load film are adjusted so that a resonance frequency of the first piezoelectric vibrator and a resonance frequency of the second piezoelectric vibrator are made different from each other.
Here, another arrangement may be made in which the thicknesses of the upper load film of the first piezoelectric vibrator and the upper load film of the second piezoelectric vibrator are made equal to each other and the thicknesses of the lower load film of the first piezoelectric vibrator and the lower load film of the second piezoelectric vibrator are made different from each other.
Moreover, the upper load film of the first piezoelectric vibrator and the upper load film of the second piezoelectric vibrator may be made from the same material. The lower load film of the first piezoelectric vibrator and the lower load film of the second piezoelectric vibrator may be made from the same material.
Furthermore, supposing that the thickness of the upper load film of each of the first and second piezoelectric vibrators is Ta, the thickness of the lower load film of the first piezoelectric vibrator is Tb, the thickness of the lower load film of the second piezoelectric vibrator is Tc, the sound velocity of the upper load film of each of the first and second piezoelectric vibrators is va, and the sound velocity of the lower load film of each of the first and second piezoelectric vibrators is vb, Ta may be made greater than Tb×va/vb, and also made smaller than Tc×va/vb.
Moreover, by respectively adjusting the thickness of the lower load film of the first piezoelectric vibrator and the thickness of the lower load film of the second piezoelectric vibrator, the resonance frequencies of the first and second piezoelectric vibrators may be adjusted;
the first and second piezoelectric vibrators are formed on the substrate with a supporting portion interposed therebetween;
the supporting portion is formed by joining a first supporting portion formed on the substrate to a second supporting portion formed on a non-adjusted area in which the thicknesses of the lower load films of the first and second piezoelectric vibrators are unadjusted; and
an adjusted area in which the thicknesses of the lower load films of the first and second piezoelectric vibrators are adjusted may be made to face the substrate, with a void portion being interposed therebetween.
Moreover, by respectively adjusting the thickness of the lower load film of the first piezoelectric vibrator and the thickness of the lower load film of the second piezoelectric vibrator, the resonance frequencies of the first and second piezoelectric vibrators may be adjusted;
the first and second piezoelectric vibrators are formed on the substrate with a supporting portion interposed therebetween;
the supporting portion is formed by a non-adjusted area in which the thicknesses of the lower load films are unadjusted, and the non-adjusted area of the lower load films and the substrate are directly joined to each other; and
an adjusted area in which the thicknesses of the lower load films of the first and second piezoelectric vibrators are adjusted may be allowed to face the substrate, with a void portion being interposed therebetween.
In accordance with the piezoelectric vibrator of the present invention, since the node of vibration mode is made closer to the center of the piezoelectric vibrator in comparison with the conventional frequency adjusting method, energy can be efficiently utilized so that a superior coupling coefficient can be achieved. Moreover, in the case where the frequency adjusting method of the present invention is applied to a transferring technique, adjustments for realizing desired frequency intervals can be carried out prior to the transferring operation (prior to formation of a void portion) so that, by reducing the number of processes after the transferring operation, degradation of the yield can be suppressed.